Runaway Greenhouse Planet
July 30, 2013

How A Planet Becomes A ‘Runaway Greenhouse’

Brett Smith for - Your Universe Online

When a planet absorbs too much solar radiation, its atmosphere reaches a tipping point - overheating, boiling its oceans and filling with steam. The result is an uninhabitable world much like Venus.

This 'runaway greenhouse' phenomenon typically occurs just inside the habitable zone orbited by planets like Earth, and a new report in Nature Geoscience indicates that reaching the tipping point may be easier than previously thought.

Study co-author and University of Victoria climate scientist Colin Groldblatt recently told NBC News that we aren't likely to see a runaway greenhouse on Earth anytime soon, despite the fact that the atmosphere now contains more of the greenhouse gas carbon dioxide than ever - some 400 parts per million, by the most recent and accurate measurements.

"Our estimate is that it would take 30,000 parts per million of carbon dioxide in the atmosphere to make it warm enough to trigger this runaway greenhouse," Goldblatt explained.

The climatologist said that the runaway greenhouse phenomenon occurs when a planet absorbs more solar radiation than it can release, much like filling a bathtub at a rate that the drain can't keep up with.

"If you turn the taps on harder, you will fill it up with more water than can get out at one any one time, then eventually your bath is going to fill up to overflowing," he explained. "And it is the same kind of process with the runaway greenhouse."

"If we absorb more solar radiation than that maximum we can emit, then all that can happen is the Earth is going to warm," he added. "It just can't keep itself in energy balance anymore."

Using new computer modeling methods, astronomers at the University of Washington and the University of Victoria in Canada have discovered a new lower thermal radiation threshold for the runaway greenhouse process to begin.

"The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse," said co-author Tyler Robinson, a UW astronomy postdoctoral researcher.

While the researchers admitted that further research is necessary, they suggested that a recalibration of where the habitable zone starts and ends might be in order. This could mean that some newly discovered planets will be deemed uninhabitable.

"These worlds on the very edge got 'pushed in,' from our perspective - they are now beyond the runaway greenhouse threshold," Robinson said.

The team said future research would be necessary because their computer models relied on a one-dimensional measure averaged around a planet that assumes zero cloud cover.

As the sun increases in brightness in the very distant future, the findings will apply to planet Earth as well, the researchers noted.

"As the solar constant increases with time, Earth's future is analogous to Venus's past," they wrote in their report.

"Venus shows us what we will be like in the future," Goldblatt said, "and it is not pretty."

Some scientists see the future Earth as primarily desert-like, marked by small pockets of water and huge salt flats in place of oceans.